1. Technical Field
The present disclosure relates to a low voltage switch. The disclosure also relates to a transmission channel of a high voltage signal to a load, in particular a piezoelectric transducer and to a corresponding driving method. The disclosure particularly, but not exclusively, relates to a low voltage switch for a transmission channel for ultrasound applications and the following description is made with reference to this field of application by way of illustration only.
2. Description of the Related Art
As it is well known, sonography or ultrasonography is a system of medical diagnostic testing that uses ultrasonic waves or ultrasounds and is based on the principle of the transmission of the ultrasounds and of the emission of echoes and is widely used in the internal medicine, surgical and radiological fields.
The ultrasounds normally used are comprised between 2 and 20 MHz. The frequency is chosen by taking into consideration that higher frequencies have a greater image resolving power, but penetrate less in depth in the subject under examination.
These ultrasounds are normally generated by a piezoceramic crystal inserted in a probe maintained in direct contact with the skin of the subject with the interposition of a suitable gel (suitable for eliminating the air between probe and subject's skin, allowing the ultrasounds to penetrate into the anatomic segment being examined). The same probe is able to collect a return signal or echo, which is suitably processed by a computer and displayed on a monitor.
In particular, the ultrasounds that reach a variation point of the acoustic impedance, and thus for example an internal organ, are partially reflected and the reflected percentage conveys information about the impedance difference between the crossed tissues. It is to be noted that, the big impedance difference between a bone and a tissue being considered, with sonography it is not possible to see behind a bone, which causes a total reflection of the ultrasounds, while air or gas zones give “shade”, causing a partial reflection of the ultrasounds.
The time employed by an ultrasonic wave for travelling across the path of transmission, reflection and return is provided to the computer, which calculates the depth wherefrom the echo has come, thus identifying the division surface between the crossed tissues (corresponding to the variation point of the acoustic impedance and thus to the depth wherefrom the echo comes).
Substantially, an ultrasonographer, in particular a diagnostic apparatus based on the ultrasound sonography, typically comprises three parts:                a probe comprising at least one transducer, in particular of the ultrasonic type, which transmits and receives an ultrasound signal;        an electronic system that drives the transducer for the generation of the ultrasound signal or pulse to be transmitted and receives an echo signal of return of this pulse at the probe, processing in consequence the received echo signal; and        a displaying system of a corresponding processed sonographic image starting from the echo signal received by the probe.        
In particular, the word transducer generally indicates an electric or electronic device that converts a type of energy relative to mechanical and physical quantities into electric signals. In a broad sense, a transducer is sometimes defined as any device that converts energy from a form to another, so that it can be re-processed either by men or by other machines. Many transducers are both sensors and actuators. An ultrasonic transducer usually comprises a piezoelectric crystal that is suitably biased for causing its deformation and the generation of the ultrasound signal or pulse.
Ultrasonic transducers for sonographic images are usually driven by high voltage driving circuits or drivers able to generate a sinusoidal signal of variable width comprised between 3 and 200 Vpp and frequencies from 1 MHz to 20 MHz, this sinusoidal signal being a control signal for corresponding generators of the ultrasound pulse to be transmitted, in particular piezoelectric crystals.
The corresponding driving circuits are thus made of components that can sustain these high voltages and that, given the frequencies at stake, can supply currents high enough to a load applied at the output, in particular an ultrasonic transducer.
As a result, prior art ultrasonic transducer driving circuits use components with rather big sizes. These components however add high parasitic capacitances in parallel to the transducer.
Moreover, the transducer itself is used also for the reception in a transmission channel for these ultrasound applications. Typically, an ultrasonic transducer transmits a high voltage pulse of the duration of a few us, and receives the echo of this pulse, generated by the reflection on the organs of a subject under examination, for the duration of about 250 us, and then returns to the transmission of a new high voltage pulse. For example, a first pulse IM1 and a second pulse IM2 are transmitted with an excursion peak to peak equal, in the example shown, to 190 Vpp with reception by the transducer of corresponding echoes, indicated with E1 and E2, as schematically shown in FIG. 1.
The echo signal or return acoustic wave is converted into an electric wave that is a signal of some millivolts of width, signal that is then amplified by low noise amplifier circuits, connected to the transducer itself, in turn disturbed by the parasitic capacitance due to the high voltage components of the driving circuit of the transducer. This reduces the quality of the echo signal.